Unit 1: Atomic Structure and Periodicity

Answer: D — The correct answer is D because an atom with an atomic number of 6 is carbon, and the atomic mass of 12 corresponds to the most common isotope of carbon, which has 6 protons, 6 electrons, and 6 neutrons. A is incorrect because the charge on an ion is determined by the number of electrons, not the atomic number or mass. B is incorrect because if the atom has 6 protons and only 5 neutrons, its atomic mass would be 11, not 12. C is incorrect because while it is possible for carbon to have 8 neutrons, this would result in an atomic mass of 14, not 12.

Answer: A — The correct answer, C, is the best explanation because as you move from left to right across a period, the number of protons in the nucleus increases. This increase in protons results in a greater effective nuclear charge, which pulls the electrons closer to the nucleus and decreases the atomic radius. Option A is incorrect because the increase in protons actually decreases the atomic radius. Option B is incorrect because the number of electron shells does increase across a period, but this is not the primary reason for the decrease in atomic radius. Option D is incorrect because the increase in electron shells would result in a larger atomic radius, not a decrease.

Answer: D — The correct answer, B, is based on the trend that elements with high atomic numbers tend to be radioactive and synthetic, often exhibiting high reactivity due to their partially filled outer energy levels. Option A is incorrect because a completely filled outer energy level is characteristic of noble gases, which are unreactive. Option C is incorrect because the element's high atomic number would place it far beyond the noble gases in the periodic table. Option D is incorrect because an element with an atomic number of 123 would have a much higher atomic mass than the nonmetals, which are generally located near the top of the periodic table.

Answer: D — The correct answer is D because an atom with 12 protons is magnesium (Mg), and since it has 12 electrons, it is a neutral atom. A is incorrect because the atom has an equal number of protons and electrons, so it is not an ion. B is incorrect because the outer energy level of magnesium is not full (it has 2 electrons in its outermost energy level, but can hold up to 8). C is incorrect because the number of orbitals is related to the energy level, not the number of protons or neutrons.

Answer: C — The correct answer is C because in a neutral atom, the total number of electrons is equal to the atomic number (number of protons), which is 15 for phosphorus. Option A is incorrect because the total number of electrons in a neutral atom cannot be less than the atomic number. Option B is incorrect because the atomic mass is the total number of protons and neutrons, not electrons. Option D is incorrect because the total number of electrons is not greater than 15 times the number of protons, but rather equal to the number of protons.